Major depressive disorder (MDD) is a common, typically recurrent and disabling disorder, costing the U.S. over $44 billion/year in direct and indirect costs, and with point prevalence rates estimated at 5%-9% for women, and 2%-3% for men. More than one third of patients suffering from MDD appear to use alternative therapies in the U.S. Routinely prescribed in Europe for nearly 30 years and released four years ago in the U.S. as an over-the-counter dietary supplement, s-adenosyl-l-methionine (SAMe) has gained significant popularity as an agent marketed for improving mood and emotional well being. A number of relatively small double-blind studies have shown that parenteral or oral preparations of SAMe, compared with a number of standard tricyclic antidepressants, were generally equally effective, and tended to produce fewer side effects A relatively smaller number of studies have also examined the efficacy of SAMe compared to placebo, with the majority of these studies showing a significant advantage of SAMe over placebo. The recent report of the Southern California Evidence-Based Practice Center for the U.S. Department of Health and Human Services [Agency for Healthcare Research and Quality (AHRQ Publication 2002; http://www.ahrq.gov) ] states that "The results of these studies justify additional randomized clinical trials to evaluate the efficacy and tolerability of SAMe for treatment of depression." No adequately powered placebo-controlled study of SAMe in depression has ever been conducted in the U.S. We therefore propose a five-year, placebo-controlled, two-site study to assess the efficacy and safety of SAMe and of a standard selective serotonin reuptake inhibitor (SSRI), escitalopram in outpatients with MDD. This proposal is a parallel comparison of the efficacy and safety of SAMe, escitalopram, and placebo, with a crossover phase during which non-responders to any of these three treatments receive open-label treatment with the combination of escitalopram and SAMe. It is important to assess the efficacy of this combination therapy because patients frequently self-medicate with SAMe during standard antidepressant treatment. The primary aim of the proposed study is to test the acute antidepressant efficacy and tolerability of both SAMe and escitalopram, each compared to placebo, for the treatment of MDD. Secondary aims are to assess the acute effects of SAMe or escitalopram vs. placebo on remission rates, quality of life, and psychosocial functioning.